This invention relates to fluid flow sensing elements for measuring gas flow rates and, more specifically, to an area averaging Pitot tube arrangement, modular in design specifically adapted for use with a cyclone style furnace air flow element used for boiler combustion air measuring and control so as to provide an improved method of installation. It is useful in measuring gas flow rates in rectangular, irregular, or ducts having non-circular cross-sectional area and in high-temperature applications. It is particularly useful in the boiler combustion systems wherein furnace retrofit projects require replacement of the combustion air flow measuring element without damaging or altering significantly the furnace structure or duct work.
The instrumentation and process control industry has recognized the use of the Pitot tube as a reliable device for measuring the volumetric flow of both liquids and gasses for many years. The Pitot tube operates based upon the principal that when a fixed probe is inserted into piping or duct work containing a moving fluid, the total pressure sensed by the probe is the sum of the static pressure exerted by the fluid, whether in motion or at rest, and the dynamic pressure equivalent to the kinetic energy of the fluid in motion. Conventional Pitot tube arrangements provide measurement of both the static and total pressure of the flowing fluid, the difference between which is the dynamic pressure. This differential pressure, i.e. the dynamic pressure, is directly related to and can be used to calculate the linear flow rate within the piping or duct work. The volumetric flow rate of the fluid is determined by multiplying the linear flow rate by the cross-sectional area of the conduit.
The Pitot tube is particularly useful in measuring gas flows in piping or duct work with a large cross-sectional area because they cause negligible pressure loss within the conduit. In application, it is well known that flow rates, and thus dynamic pressures, within a conduit are not uniform. Affected by variables such as the Reynolds number of the particular gas and turbulence caused by surface roughness, dampers, elbows and other fittings, the flow rate/dynamic pressure is generally higher toward the center of the conduit and lower towards the outer extremes. This phenomenon is described in terms of a velocity profile, wherein a vector representation of the linear velocities at various points within the conduit defines a characteristic profile curve. The dynamic nature of the velocity profile precludes accurate measurement with a single Pitot tube. Rather, an accurate measurement of the flow within the conduit is obtained by placing the Pitot tubes at various positions on a cross-sectional plane, sampling the dynamic pressure at various points across the velocity profile, averaging them, and using the result to calculate a volumetric flow rate.
Presently, area averaging Pitot tube arrays consist of a fluid flow element wherein an inlet and an outlet is provided having a housing with the same internal dimensions as the fluid conduit. An interior flow conditioner, affixed at the inlet of the flow element, helps to minimize turbulence and produce a more uniform velocity profile. A total pressure sensing Pitot tube array traverses the interior cross-sectional area of the flow element for sensing the total pressure of fluid flowing therethrough. A static pressure sensing Pitot tube array traverses the interior cross-sectional area of the flow element for sensing the average static pressure therein. Each Pitot tube array is equipped with a common header that serves to average the individual Pitot tube pressures. Exterior instrument taps, connected to the respective common headers, are provided for connection of each array to a differential pressure instrument for indicating flow rate and/or transmitting a flow rate signal. Using the aforementioned principles, this signal is used to calculate the volumetric flow rate through the element.
According to the preferred embodiment of the present invention, an improved fluid flow element incorporating the use of area averaging Pitot tube arrays is provided. The element contains an inlet and an outlet in a housing with the same internal dimensions as that of the fluid conduit. The element also contains an interior flow conditioner and a bell mouth inlet to direct the flow into the element while minimizing entrance pressure drop at the inlet side of the element. The element is modular in construction so as to allow installation in furnace retrofit applications where the size of furnace access doors precludes the installation of a one-piece element. The element is comprised of equal sized sections, each of which is equipped with a total pressure Pitot tube array and a static pressure Pitot tube. The modular design allows for the use of shorter Pitot tubes, thus reducing undesirable effects of harmonic vibration and differential expansion caused by high fluid velocity and temperature, respectively. Common headers, equipped with individual instrument taps, allow for sectional flow measurement in each quadrant. Once the modules are installed, field piping is used to connect the total and static Pitot tube arrays, respectively, resulting in a single pair of instrument taps for flow determination across the entire element.